Speed change control apparatus for continuously variable transmission

ABSTRACT

A continuously variable transmission having a primary pulley and a secondary pulley on which a belt wound on the primary pulley is fitted. The width of the groove on the primary pulley is adjusted by the oil pressure in the primary hydraulic chamber in the primary cylinder, the width of the groove on the secondary pulley is adjusted by the oil pressure in the secondary hydraulic chamber in the secondary cylinder. The switching operation between the speed change operation in a state in which the line pressure is supplied to the secondary hydraulic chamber and the primary pressure is supplied to the primary hydraulic chamber by the oil passage switching valve, and the speed change control in a state in which the line pressure is supplied to the primary hydraulic chamber and the primary pressure is supplied to the secondary hydraulic chamber can be performed.

BACKGROUND OF THE INVENTION

The present invention relates to a speed change control apparatus forbelt-driven continuously variable transmissions having a single cylinderas a primary cylinder for driving a primary pulley.

Conventionally, some of the belt-driven continuously variabletransmissions (hereinafter, referring CVT) for automotive vehicles areconstructed in such a manner that a metallic belt is wound between aprimary (driving) pulley and a secondary (driven) pulley. The primarypulley is mounted on a primary shaft on the driving side, and has apulley groove of a variable width. The secondary pulley is mounted on asecondary shaft on the driven (or following) side, and has a pulleygroove of a variable width. Pulley diameters of the primary pulley andthe secondary pulley may be varied by hydraulic pressure so as tocontinuously change the number of rotation of the secondary shaft.

The speed change control of the CVT is carried out by controlling thehydraulic pressure supplied to the primary cylinder provided on theprimary pulley and the secondary cylinder provided on the secondarypulley. The hydraulic pressure supplied to the respective cylinders aregenerated by the oil pump driven by an engine. The line pressuresupplied to the secondary cylinder, or the secondary pressure, isadjusted by the line pressure-adjusting valve, and the primary pressuresupplied to the primary cylinder is adjusted by the primarypressure-adjusting valve with a line pressure as an original pressure.By adjusting the primary pressure to the value corresponding to thetarget speed change ratio or the speed-change velocity, the width of thegroove of the primary pulley is changed to control the vehicle speed,and the line pressure is adjusted to the value corresponding to thetransmissible capacity required to the belt.

Since the CVT of such a hydraulic system is adapted to adjust theprimary pressure by depressurizing the line pressure, the primarypressure never exceeds the line pressure. Therefore, the effective areaof the primary cylinder is determined to be larger than, about twicethat of the secondary cylinder, since the up-shifting will be difficultunless otherwise the effective area of the primary cylinder is largerthan the effective area of the secondary cylinder, in order to controlthe speed change by a hydraulic force of the primary cylinder.

Therefore, when the primary cylinder consists of a single cylinder, thediameter of the primary cylinder have to be larger than that of thesecondary cylinder, which results in increase in the capacity of inertiaof the primary cylinder.

Attempts have been made in the related art to construct the primarycylinder in a duplicated structure, or in double-cylindered structure,as in Japanese Patent Laid-Open No. 196749/1998. According to thistechnology, the effective area of the primary cylinder can be securedwithout increasing the diameter of the cylinder. However, the structureof the primary cylinder becomes complex, which results in increase incost.

SUMMARY OF THE INVENTION

Accordingly, it is the object of the present invention to provide aprimary cylinder in a single structure, while maintaining the smalldiameter of the primary cylinder.

The object can be achieved by a speed change control apparatus forcontinuously variable transmissions, according to the present invention,having a primary pulley to be mounted on a primary shaft and having apulley groove with a variable width, a secondary pulley to be mounted ona secondary shaft and having a pulley groove with a variable width, abelt wound on both of that primary pulley and that secondary pulley, aprimary cylinder mounted on the primary pulley and provided with aprimary hydraulic chamber, and a secondary cylinder mounted on thesecondary pulley and provided with a secondary hydraulic chamber. Thespeed change control apparatus comprises:

a line pressure adjusting valve for adjusting hydraulic fluid suppliedfrom a oil pump into a line pressure;

a primary pressure adjusting valve for adjusting the line pressure intoa primary pressure; and

oil passage switching mechanism for switching an oil passage between afirst condition and a second condition, wherein in the first condition,the line pressure is supplied to the secondary hydraulic chamber and theprimary pressure is supplied to the primary hydraulic chamber, andwherein in the second condition, the line pressure is supplied to theprimary hydraulic chamber and the primary pressure to the secondaryhydraulic chamber.

In the speed change control apparatus for continuously variabletransmissions, according to the present invention, it is advantageousthat the effective area of the primary cylinder is determined to bealmost the same as the effective area of the secondary cylinder, andalso that the oil passage switching means switches the oil passage inthe region of speed change ratio in which the belt winding diameters forthe primary pulley and for the secondary pulley are almost the same.

Further, in the speed change control apparatus for continuously variabletransmissions, according to the present invention, it is advantageousthat the effective area of the primary cylinder is determined to be 5 to60% larger than the effective area of the secondary cylinder.

Furthermore, in the speed change control apparatus for continuouslyvariable transmissions according to the present invention, it ispreferable to further comprise a controller for carrying out a speedchange control in such a manner that the oil pressure to be supplied tothe primary hydraulic chamber is adjusted in the low-speed region andthat the oil pressure to be supplied to the secondary hydraulic chamberis adjusted in the high-speed region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of the drive systemfor the belt-driven continuously variable transmissions;

FIG. 2 is a hydraulic circuit diagram for performing the speed changeoperation;

FIG. 3 is a block diagram showing a speed change control circuit; and

FIGS. 4A and 4B are timing charts showing the relation between the oilpressures of the primary and secondary pulley hydraulic chambers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, an embodiment of the present inventionwill be described.

FIG. 1 is a schematic drawing showing an example of the belt-drivencontinuously variable transmission, or the CVT driving system, in whichthe rotation of a crank shaft 1 driven by an engine, which is not shown,is transmitted to a continuously variable transmission mechanism 4 via atorque converter 2 and a forward and reverse switching mechanism 3.

The torque converter 2 has a lockup clutch 5, which is connected to aturbine shaft 6. An apply chamber 7 a is provided on one end of thelockup clutch 5, and a release chamber 7 b is provided on the other endthereof. The torque converter 2 is brought into operation by circulatingthe oil pressure supplied to the release chamber 7 b via the applychamber 7 a. In contrast to it, the lockup clutch 5 is engaged with afront cover 8 and thus brought into a locked-up state by supplying theoil pressure to the apply chamber 7 a and lowering the oil pressure inthe release chamber 7 b. The slip pressure control is performed to slipthe lockup clutch 5 by adjusting the pressure in the release chamber 7b.

The forward and reverse switching mechanism 3 comprises a forward clutch11 (as a clutch for the forward movement) for transmitting the rotationof the turbine shaft 6 as an output shaft of the torque converter 2 tothe continuously variable transmission mechanism 4 in the forwarddirection, and a reverse brake 12 (as a brake for reverse movement) fortransmitting the same in the reverse direction. When an oil pressure issupplied to the clutch hydraulic chamber 11 a and the forward clutch 11is brought into connection, the rotation of the turbine shaft 6 istransmitted to the continuously variable transmission mechanism 4 in theforward direction, and when a oil pressure is supplied to the brakehydraulic chamber 12 a and the reverse brake 12 is brought intoconnection, it is braked and transmitted in the reverse direction.

The continuously variable transmission mechanism 4 comprises an inputshaft (i.e., the primary shaft 13) to be connected to the forward andreverse switching mechanism 3, and an output shaft (i.e., the secondaryshaft 14) extended in parallel with the input shaft. The primary shaft13 is provided with a primary pulley 15. The primary pulley 15 has astationary pulley 15 a fixed on the primary shaft 13 and a movablepulley 15 b axially slidably fitted on the opposite side of the primaryshaft 13 by a ball spline or the like. Accordingly, the distance betweenthe conical surfaces of the pulley, or the width of the pulley groovecan be varied.

The secondary shaft 14 is provided with a secondary pulley 16. Thesecondary pulley 16 has a stationary pulley 16 a fixed on the secondaryshaft 14 and a movable pulley 16 b axially and slidably fitted on theopposite side of the secondary shaft 14 in the same manner as themovable pulley 15 b. Accordingly, the width of the pulley groove can bevaried.

A belt 17 is wound between the primary pulley 15 and the secondarypulley 16, so that the rotation of the primary shaft 13 is changed inspeed in continuously variable manner and is transmitted to thesecondary shaft 14 by changing widths of the grooves on both of thepulleys 15, 16 and a ratio between the winding diameters for the pulleys15 and 16.

The rotation of the secondary shaft 14 is transmitted to the wheels 19a, 19 b via a gear train including a reduction gear and a differentialapparatus 18. In the case of front-wheel-drive vehicles, the wheels 19a, 19 b are front wheels. The basic structure of the above-describeddriving system of the CVT is disclosed, for example, in Japanese PatentLaid-Open No.325458/1998.

In order to change the width of the groove on the primary pulley 15, aplunger 21 having a cylindrical portion and a disk portion is fixed onthe primary shaft 13, and a primary cylinder 22 which slidably contactswith the outer peripheral surface of the plunger 21 is fixed on themovable pulley 15 b, and a primary hydraulic chamber 23 is formedbetween the plunger 21 and the movable pulley 15 b.

In order to change the width of the groove on the secondary pulley 16, aplunger 26 having a tapered cylindrical portion is fixed on thesecondary shaft 14, the secondary cylinder 27 which slidably contactswith the outer peripheral surface of the plunger 26, and a secondaryhydraulic chamber 28 is formed between the plunger 26 and the movablepulley 16 b. In the case shown in FIG. 1, the diameter of the primarycylinder 22 is almost the same as that of the secondary cylinder 27, andboth of the effective areas are determined to be almost the same.

When the hydraulic fluid is supplied to the primary hydraulic chamber 23in the primary cylinder 22 to increase the capacity thereof, the movablepulley 15 b moves toward the stationary pulley 15 a together with thecylinder 22, and thus the width of the pulley groove is lessened, andwhen the capacity is reduced, the width of the pulley groove increases.When the operation hydraulic fluid is supplied to the secondaryhydraulic chamber 28 in the secondary cylinder 27 to increase thecapacity thereof, the movable pulley 16 b moves toward the stationarypulley 16 a together with the cylinder 27 so that the width of thegroove on the pulley is reduced, and when the capacity is reduced, thewidth of the pulley groove increases.

FIG. 2 is a circuit diagram of the hydraulic pressure for supplying thehydraulic fluid to the primary cylinder 22 and the secondary cylinder 27to carry out the speed change operation. The outlet port of the oil pump30 to be driven by an engine is connected to the pressure adjusting portof the line pressure adjusting valve 32 via a line pressure passage 31,and the line pressure (secondary pressure) is adjusted by the linepressure adjusting valve 32 according to the input torque to the CVT orthe speed change ratio.

The line pressure passage 31 is connected to the input port of theprimary pressure adjusting valve 33, which adjusts the hydraulicpressure in the primary pressure passage 34 to a primary pressure usingthe line pressure as an original pressure. The primary pressure isadjusted according to the velocity of the vehicle, the number ofrotation of the primary pulley, and the extent of opening of thethrottle.

Both of the line pressure adjusting valve 32 and the primary pressureadjusting valve 33 are proportional valves, which can adjust a pressureto a desired value with in a prescribed range of pressure.

A primary line 35 is connected to the primary hydraulic chamber 23 inthe primary cylinder 22 and a secondary line 36 is connected to thesecondary hydraulic chamber 28 in the secondary cylinder 27.

The oil passage shown in FIG. 2 is provided with a switchover valve 37to switch between two positions (A) and (B) when the solenoid 37 a isenergized. The switchover valve 37 switches between the position (A) inwhich the line pressure is supplied to the secondary hydraulic chamber28, and the primary pressure is supplied to the primary hydraulicchamber 23 by establishing communication between the line pressurepassage 31 and the secondary line 36 and between the primary pressurepassage 34 and the primary line 35, and the position (B) in which theline pressure is supplied to the primary hydraulic chamber 23 and theprimary pressure is supplied to the secondary hydraulic chamber 28 byestablishing communication between the line pressure passage 31 and theprimary line 35 and between the primary pressure passage 34 and thesecondary line 36.

A lubricant pressure adjusting valve 38 is provided between the drainport of the line pressure adjusting valve 32 and the inlet of the oilpump 30, so that the oil with lubricant pressure supplied to thelubricant pressure passage 39 is adjusted with the drain pressure of theline pressure adjusting valve as an original pressure, and the hydraulicfluid at the lubricant pressure is supplied to the lubricating portionof the forward and reverse switching mechanism 3, the lubricatingportion of the belt 17, and the like. A clutch pressure adjusting valve,which is not shown in the figure, is connected to the line pressurepassage 31, so that the clutch pressure is adjusted by the clutchpressure adjusting valve while using the line pressure as an originalpressure. The hydraulic fluid at the clutch pressure is supplied to theclutch hydraulic chamber 11 a of the forward clutch 11 and the brakehydraulic chamber 12 a of the reverse brake 12 of the forward andreverse switching mechanism 3, and to the apply chamber 7 a of thelock-up clutch 5.

The oil pressure circuit for supplying the hydraulic fluid at the clutchpressure and the lubricant pressure to the forward and reverse switchingmechanism 3 is the same as the one disclosed in the publication ofJapanese Patent Laid-Open No. 325458/1998 described above.

FIG. 3 is a block diagram showing the speed change control circuit. Thecontroller 40 having a central processing unit or a memory is connectedwith range detecting means 41 for detecting a prescribed range selectedby a selecting lever, vehicle velocity detecting means 42 for detectingthe velocity of the vehicle, throttle opening detecting means 43 fordetecting the extent of opening of the throttle valve, and the primarypulley speed detecting means 44 for detecting the number of rotation ofthe primary pulley 15, so that the control signals are fed to thesolenoid 32 a of the line pressure adjusting valve 32, the solenoid 33 aof the primary pressure adjusting valve 33, and the solenoid 37 a of theoil passage switching valve 37 based on the detected signals.

FIG. 4A and FIG. 4B are timing chart showing the relation between theoil pressure in the primary hydraulic chamber 23 and the oil pressure inthe secondary hydraulic chamber 28 according to the pulley ratio, or thespeed change ratio, and the timing of the operation of the oil passageswitching valve 37. In this timing chart, the area on the left side ofthe intermediate speed change ratio (MID), or the speed change ratio inwhich the belt winding diameter of the primary pulley 15 and the beltwinding diameter of the secondary pulley 16 are the same (speed changeratio i=1), is a low speed stage, or LOW stage, and the area on theright side thereof is a high speed stage, or the over drive stage.

The line pressure can always be set to a desired value between zero andthe maximum line pressure by the line pressure-adjusting valve 32. Asshown in FIG. 4A and FIG. 4B, in the LOW stage area, the line pressureis introduced as an oil pressure in the secondary hydraulic chamber 28,so that the speed change control is performed by adjusting the oilpressure in the primary hydraulic chamber 23, or the primary pressure,in the range from zero to the line pressure. On the other hand, in theoverdrive stage area, the line pressure is introduced as an oil pressurein the primary hydraulic chamber 23 and the primary pressure isintroduced in the secondary hydraulic chamber 28 so that the speedchange control is performed by adjusting the primary pressure in therange from zero to the line pressure.

The oil passage switching valve 37 supplies the line pressure to thesecondary hydraulic chamber 28 in the area in which the pulley ratio issmaller than the MID point, or the LOW-side stage in which the speedchange ratio is large, and is set to the position where the primarypressure is supplied to the primary hydraulic chamber 23. At thismoment, the belt winding diameter of the primary pulley 15 is smallerthan that of the secondary pulley 16, and the primary pressure isadjusted by the primary pressure adjusting valve 33 within the rangebetween zero and the lowest pressure (approx. 0.5 Mpa) at the LOWposition. For example, when the primary pressure is zero, the oilpressure in the primary hydraulic chamber 23 is reduced to zero, and aclamping force of the primary pulley 15 becomes smaller than theclamping force of the secondary pulley 16, so that the pulley ratio isin the state of LOW position. When the primary pressure supplied to theprimary hydraulic chamber 23 of the primary cylinder 22 is increased bythe primary pressure adjusting valve 33 from the state of LOW position,the belt winding diameter of the primary pulley 15 increases, and thusthe speed change ratio is changed to the MID side, or the intermediatespeed change ratio, to perform the up-shift speed change.

On the other hand, when the primary pressure and the line pressure arealmost the same, the effective areas of the primary cylinder 22 and thesecondary cylinder 27 are set to the identical value, thus the clampingforces of the primary pulley 15 and the secondary pulley 16 becomealmost the same and the pulley ratio becomes the MID state. When theprimary pressure is set to a desired value within the range from zero tothe line pressure, it is stabilized at the prescribed pulley ratiowithin the range between LOW and MID due to the relation between theclamping forces of both of the pulleys.

When the primary pressure is slightly shifted toward the line pressurewhile maintaining the line pressure in the stabilized state, theclamping force of the primary pulley 15 increases by the extentcorresponding to the slightly shifted oil pressure, and as a consequent,the relation between the primary clamping force and the secondaryclamping force looses its stability, and the pulley ratio is shiftedtoward the MID position, and then the relation is stabilized again uponcompletion of the shift, thereby accomplishing the up-shift operation.

When the primary pressure is slightly shifted toward zero while the linepressure is maintained in the stabilized state, the clamping force ofthe primary pulley 15 decreases by the extent corresponding to theslightly shifted oil pressure, and as a consequent, the relation betweenthe clamping forces of both of the pulleys looses its stability, and thepulley ratio is shifted toward the LOW position, and then the relationis stabilized again upon completion of the shift, thereby accomplishingthe down-shift operation.

Being increased to the value equivalent to the line pressure by theup-shift operation, the primary pressure increases to the MID positionin which the speed change ratio i=1. However, since the line pressure isused as the original pressure of the primary pressure, further up-shiftoperation cannot be made under the primary pressure.

Therefore, the switching operation is made by energizing the solenoid 37a of the oil passage switching valve 37, so that the line pressure issupplied to the primary hydraulic chamber 23 and the primary pressure issupplied to the secondary hydraulic chamber 28. As a consequent, thespeed change control from the MID position to the OD at which the speedchange ratio is the smallest can be made by adjusting the primarypressure supplied to the secondary hydraulic chamber 28.

In other words, since the connection between the respective adjustingvalves 32, 33 and the pulley cylinder under the speed change ratio fromMID to OD is the reverse of the case from LOW position to MID position,the primary pressure is supplied to the secondary hydraulic chamber 28and the line pressure is supplied to the primary hydraulic chamber 23.Though the oil pressure in the primary hydraulic chamber 23 is nowequivalent to the line pressure, the primary pressure to be adjusted bythe primary pressure adjusting valve 33 can be adjusted in the rangefrom zero to the maximum line pressure. For example, assuming that theprimary pressure is zero, the oil pressure of the secondary hydraulicchamber 28 can be reduced to zero with the line pressure supplied to theprimary hydraulic chamber 23. However, the oil pressure in the secondaryhydraulic chamber 28 is not actually reduced to zero in order togenerate the belt clamping force.

As regards the clamping force of the primary pulley 15 and the clampingforce of the secondary pulley 16, the primary side may be set to thelarger value than the secondary side. Therefore, the pulley ratio is setto the OD state. On the other hand, when the primary pressure and theline pressure are set to the same value, the pulley ratio will bebrought into the MID state as described above. By setting the primarypressure is set to the desired value within the range of the linepressure, the pulley ratio can be stabilized in a constant value withinthe range between MID and OD due to the relation between the clampingforce of the primary cylinder 22 and the clumping force of the secondarycylinder 27.

When the primary pressure is slightly shifted toward zero whilemaintaining the line pressure in the stabilized state, the clampingforce of the secondary pulley 16 decreases by the extent correspondingto the slightly shifted oil pressure, and as a consequent, the relationbetween the clamping force of the primary pulley 15 and the clampingforce of the secondary pulley 16 looses its stability, and the pulleyratio is shifted toward the OD state, and then the relation isstabilized again upon completion of the shift, thereby accomplishing theup-shift operation.

When the primary pressure is slightly shifted toward the line pressurewhile maintaining the line pressure in the stabilized state describedabove, the clamping force of the secondary pulley 16 increases by theextent corresponding to the slightly shifted oil pressure, and as aconsequent, the relation between the clamping forces of the primarypulley 15 and of the secondary pulley 16 looses its stability, and thepulley ratio is shifted toward the MID, and then the relation isstabilized again upon completion of the shift, thereby accomplishing thedown-shift operation.

In the above described case, the switching operation of the oil passageis performed by the oil passage switching valve 37 when the effectivediameters of the primary cylinder 22 and of the secondary cylinder 27are set to the same value and, in turn the belt winding diameters of theprimary pulley 15 and the secondary pulley 16 are almost the same.However, it is also possible to set the effective diameter of theprimary cylinder 22 to the larger value than the effective diameter ofthe secondary cylinder 27, so that the switching operation is performedat the position slightly shifted from the MID position shown in FIGS. 4Aand B toward the OD side. In such a case, the primary cylinder 22 isactive in terms of speed change control, but when considering theincrease in the capacity of inertia of the primary cylinder 22, it ispreferable to increase the effective diameter of the primary cylinder 22within the range from b 5 l to b 60% of the effective diameter of thesecondary cylinder 27.

The present invention is not limited to the embodiment described above,but it will be understood that the changes and variations may be madewithout departing from the scope of the invention. For example, asregards the drive system for belt-driven continuously variabletransmissions, the present invention may be applied not only to the caseshown in FIG. 1, but also to various types such as a type having notorque converter 2.

While there has been described in connection with the preferredembodiment of the invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is aimed, therefore, to cover inthe appended claim all such changes and modifications as fall within thetrue spirit and scope of the invention.

According to the present invention, since the speed change control in astate in which the line pressure is supplied to the secondary hydraulicchamber and the primary pressure is supplied to the primary hydraulicchamber and the speed change control in a state in which the linepressure is supplied to the primary hydraulic chamber and the primarypressure is supplied to the secondary hydraulic chamber can be switched,the diameter of the primary cylinder can be miniaturized.

The effective diameter of the primary cylinder can be set to almost thesame value as the effective diameter of the secondary cylinder.

Since the diameter of the primary cylinder can be reduced, the capacityof inertia thereof can be reduced, thereby increasing the speed changeresponsivity.

Since the primary cylinder can be constructed as a single cylinder, themanufacturing cost of the continuously variable transmission can bereduced.

What is claimed is:
 1. A speed change control apparatus for acontinuously variable transmission for a vehicle having, a driving wheelprovided under said vehicle for driving thereof by a driving powerthrough a crankshaft from an engine, a primary shaft mechanically andcoaxially connected to said crankshaft for receiving said driving power,a primary pulley provided on said primary shaft for axially changing awidth, a secondary shaft provided in parallel with said primary shaft, asecondary pulley provided on said secondary shaft for transmitting saidpower through a belt wound on both of said pulleys and for changing eachrotating speed depending on driving conditions of said vehicle, aprimary cylinder formed on an outer periphery of one side of saidprimary pulley for including a primary oil chamber, a secondary cylinderformed on said outer periphery of one side of said secondary pulley forincluding a secondary oil chamber, an oil pump mounted on said vehiclefor supplying an actuating fluid to said primary and secondary oilchambers via a pressure line, said speed change control apparatuscomprising: a line pressure control valve interposed in said pressureline between said oil pump and said primary oil chamber for adjusting anoil pressure therebetween to a line pressure; a primary pressure controlvalve interposed in said pressure line between said line pressurecontrol valve and said primary oil chamber for adjusting an oil pressuretherebetween to a primary pressure; and a switchover valve interposed insaid pressure line between said oil pump and said secondary cylinder forswitching to supply said actuating fluid of said line pressure to saidsecondary oil chamber and to supply said actuating fluid of said primarypressure and of said line pressure as much as required for shifting tosaid primary oil chamber when said vehicle runs at a low speed and forswitching to supply said actuating fluid of said line pressure to saidprimary pressure oil chamber and to supply said actuating fluid of saidprimary pressure and of said line pressure as much as required forshifting to said secondary oil chamber when said vehicle runs at a highspeed so as to effectively decrease an inertia capacity of said primarycylinder by decreasing a diameter of said a primary cylinder thereby toimprove responsiveness of a speed change.
 2. The speed change controlapparatus according to claim 1, wherein an effective area of the primarycylinder is substantially equal to an effective area of the secondarycylinder, and wherein the switchover valve switches to change saidsupply within a predetermined region of a speed change ratio where abelt winding diameter of said primary pulley becomes substantially equalto a belt winding diameter of said secondary pulley.
 3. The speed changecontrol apparatus according to claim 2, wherein an effective area of theprimary cylinder is substantially equal to an effective area of thesecondary cylinder, and wherein the switchover valve switches to changesaid supply when a speed change ratio defined by belt winding diametersof said primary and secondary pulleys is within a predetermined value.4. The speed change control apparatus according to claim 3, wherein theswitchover valve switches to change said supply when the belt windingdiameter of said primary pulley becomes substantially equal to the beltwinding diameter of said secondary pulley.
 5. The speed change controlapparatus according to claim 1, wherein an effective area of the primarycylinder is set to be 5 to 60% larger than an effective area of thesecondary cylinder.
 6. The speed change control apparatus according toclaim 1, further comprising: a controller for carrying out a speedchange control in such a manner that the oil pressure to be supplied tothe primary oil chamber is adjusted in a low-speed region and that theoil pressure to be supplied to the secondary oil chamber is adjusted ina high-speed region.
 7. A vehicle equipped with a continuously variabletransmission said vehicle having a driving wheel provided under saidvehicle for driving by driving power supplied through a crankshaft froman engine, a primary shaft mechanically and coaxially connected to saidcrankshaft for receiving said driving power, a primary pulley providedon said primary shaft for axially changing a width, a secondary shaftprovided in parallel with said primary shaft, a secondary pulleyprovided on said secondary shaft for transmitting said power through abelt wound on both of said pulleys and for changing each rotating speeddepending on driving conditions of said vehicle, a primary cylinderformed on an outer periphery of one side of said primary pulley forincluding a primary oil chamber, a secondary cylinder formed on saidouter periphery of one side of said secondary pulley for including asecondary oil chamber, an oil pump mounted on said vehicle to supply anactuating fluid to said primary and secondary oil chambers via apressure line, and a speed change control apparatus comprising: a linepressure control valve interposed in said pressure line between said oilpump and said primary oil chamber for adjusting an oil pressuretherebetween to a line pressure; a primary pressure control valveinterposed in said pressure line between said line pressure controlvalve and said primary oil chamber for adjusting an oil pressuretherebetween to a primary pressure; and a switchover valve interposed insaid pressure line between said oil pump and said secondary cylinder forswitching the supply of said actuating fluid of said line pressure tosaid secondary oil chamber and to supply said actuating fluid with saidprimary pressure to said primary oil chamber when said vehicle runs at alow speed and for switching the supply of said actuating fluid of saidline pressure to said primary oil chamber and to supply said actuatingfluid of said primary pressure to said secondary oil chamber when saidvehicle runs at a high speed so as to effectively decrease an inertiacapacity of said primary cylinder by decreasing a diameter of said aprimary cylinder thereby to improve responsiveness of a speed change. 8.The vehicle according to claim 7, wherein an effective area of theprimary cylinder is substantially equal to an effective area of thesecondary cylinder, and wherein the switchover valve switches to changesaid supply within a predetermined region of a speed change ratio wherea belt winding diameter of said primary pulley becomes substantiallyequal to a belt winding diameter of said second pulley.
 9. The vehicleaccording to claim 8, wherein an effective area of the primary cylinderis substantially equal to an effective area of the secondary cylinder,and wherein the switchover valve switches to change said supply when aspeed change ratio defined by belt winding diameters of said primary andsecondary pulleys is within a predetermined value.
 10. The vehicleaccording to claim 9, wherein the switchover valve switches to changesaid supply when the belt winding diameter of said primary pulleybecomes substantially equal to the belt winding diameter of saidsecondary pulley.
 11. The vehicle according to claim 7, wherein aneffective area of the primary cylinder is set to be 5 to 60% larger thanan effective area of the secondary cylinder.
 12. The vehicle accordingto claim 7, further comprising: a controller for carrying out speedchange control in such a manner that the oil pressure to be supplied tothe primary oil chamber is adjusted in a low-speed region and that theoil pressure to be supplied to the secondary oil chamber is adjusted ina high-speed region.